Understanding interactions that determine the fate of microplastics in riverbed environments

Microplastics (MPs) are an emerging contaminant of increasing concern that are ubiquitous within freshwater and marine ecosystems. Rivers are recognised as a fundamental transport pathway for MPs; connecting terrestrial plastic sources to marine ecosystems, as well as an area where high levels of biological activity and modification can occur. However, there is little consideration as to the sources and fate of plastics within these freshwater ecosystems. Rivers are subject to plastic pollution from both point (i.e. sewage systems) and diffuse (i.e. agricultural and urban runoff) sources. It is expected that riverbed sediments act as a sink for microplastic debris (1). However, the extent to which riverbeds interact with MPs and their entrapment rates will be governed by many physical, biological and chemical factors. Colonisation studies of plastic debris by microbial biofilms have shown to cause buoyant polymers to sink (2,3). Equally, microbial biofilms over riverbed sediment will influence MP infiltration and settling rates. Nevertheless, the relative importance of these processes remains largely unclear with empirical data urgently needed to parametrise models. In this project you will investigate the interactions and feedbacks between riverbed dynamics and MPs. The main aim will be to determine the key variables which contribute to the entrapment and resuspension of MPs within this freshwater ecotone. Different types of plastic particles with different densities will be considered and analysed using our newly developed method to track the movement of MPs within our laboratory-based system (4) using fluorescence-based technology and innovative methods. The release rate and sources of MPs is vital for a more complete understanding and assessment of the hazards posed by these contaminants. As such, the new insights offered by the project have the potential to contribute directly towards new policies relating to water management and environmental conservation.

Methodology:
We will use our novel mesocosm systems to investigate and isolate the different mechanistic processes governing the interaction between the riverbed and MPs. We will test different plastic polymers, with a range of densities and sizes, across a range of riverbed systems with unique characteristics (i.e. pore size, biofilm coated, bedform shape). In addition, we will adopt our newly developed method and baseline the MPs results against traditional regulatory approved fluorometric solute tracing techniques. It is important to apply our understanding to the real-world environment. As such, we will also use local rivers and sites across London as a 'living laboratory' to collect sediment cores from the river bed & perform regulatory approved tracing tests. Using FTIR (Fourier-transform infrared microspectroscopy) at the Centre for Ecology & Hydrology (CEH) in Wallingford, you will identify the types and distribution of MPs within sediments and relate this to the environment/season in which it was collected and mesocosm data generated.